Semiconductor photodetector with internal reflector

ABSTRACT

A photodetector comprises a semiconductor substrate with entrance and reflecting faces formed at the substrate upper surface. The reflecting face forms an acute angle with the substrate surface and is positioned so that an optical beam transmitted through the entrance face into the substrate is internally reflected from the reflecting face toward the substrate upper surface. A photodetector active region is formed on the substrate upper surface and is positioned so that the reflected optical beam impinges on the active region. The photodetector may be mounted on a second substrate for receiving an optical beam from a planar waveguide formed on the second substrate or an optical fiber mounted in a groove on the second substrate.

RELATED APPLICATIONS

This application is a continuation of U.S. non-provisional applicationSer. No. 10/661,709 filed Sep. 12, 2003 (now U.S. Pat. No. 6,992,276),which in turn claims benefit of U.S. provisional App. No. 60/417,805filed Oct. 10, 2002, said provisional and non-provisional applicationsbeing hereby incorporated by reference as if fully set forth herein.

BACKGROUND

The field of the present invention relates to semiconductorphotodetectors. In particular, a semiconductor photodetector isdescribed herein that includes an internal reflector.

FIGS. 1A and 1B illustrate a generic configuration including a planarwaveguide 120 on a waveguide substrate 101. A surface-mountedphotodetector 110 is placed on the waveguide substrate 101 (eitherdirectly, or on alignment/support members thereon) for detecting opticalpower propagating from an output face of waveguide 120. FIGS. 1C and 1Dillustrate another generic configuration including an optical fiber 150received in an alignment groove 152 for illuminating a photodetector 110(surface-mounted on the groove substrate 151, as in FIGS. 1C and 1D, orfabricated directly on the groove substrate). Reasons for using aphotodetector in such circumstances are numerous. For example, theoptical power propagating through waveguide 120 or fiber 150 maycomprise an optical telecommunications signal modulated at high datarates (10 or more Gbits/sec, for example), and a high-speedphotodetector 110 may be employed as a receiver for converting theoptical signal into an electronic signal. In another example, theoptical power propagating through waveguide 120 or fiber 150 maycomprise a portion of the output of a semiconductor laser or other lightsource split from the main optical output for monitoring purposes. Theresulting signal from the photodetector may be used for signalnormalization, as a feedback control signal for stabilizing theoperation of the light source, and/or for other purposes. In this typeof application a high-speed photodetector may or may not be required.Many other circumstances may be envisioned wherein detection of opticalpower propagating through an optical waveguide or an optical fiber maybe useful.

Silicon is a commonly-used planar waveguide substrate, typicallyprovided with a silica buffer layer and one or more silica-based planarwaveguides fabricated on the silica buffer layer (so-called PlanarWaveguide Circuits, or PLCs). Such substrate may also be readilyprovided with grooves for receiving an end of an optical fiber. It isoften the case (in telecommunications devices) that the wavelength ofthe optical power carried by waveguide 120 or fiber 150 lies in the 1.3μm to 1.6 μm region, for which silicon-based photodetectors are notsuitable. Photodetectors based on III-V semiconductors are suitable forthis wavelength region, but the materials are not compatible forfabrication of the photodetector directly on a silicon or silicasurface. Even if waveguide substrate and detector materials arecompatible, it may nevertheless be desirable for providing thesemiconductor photodetector as a separate component for later assemblyfor other reasons (incompatible processing steps, design flexibility,customization of waveguide and/or photodetector, and so forth). Aseparately fabricated semiconductor photodetector 110 (III-V orotherwise) is therefore often assembled onto substrate 101 or 151(silicon or otherwise) and aligned for receiving and detecting at leasta portion of the optical power propagating through waveguide 120 orfiber 150. The present disclosure addresses suitable fabrication and/oradaptation of semiconductor 110 for enabling and/or facilitating suchassembly.

For mounting on a substantially planar substrate 101 or 151, it isadvantageous for photodetector 110 to also be fabricated/mounted on itsown substantially planar substrate. The light to be detected propagatessubstantially parallel to these planar substrates. However, the layersthat form the photodetector active region on the substrate are alsosubstantially parallel to the substrates, rendering absorption anddetection of the light by the photodetector problematic in many cases.Redirection of the light out of a plane parallel to the substratesfacilitates detection thereof. A photodetector implemented according tothe present disclosure employs internal reflection from an angled faceof the photodetector substrate for directing the light toward the activeregion thereof.

SUMMARY

A photodetector comprises a photodetector substrate with angled entranceand reflecting faces formed at the substrate upper surface. Thereflecting face forms an acute angle with the substrate upper surfaceand is positioned relative to the entrance face so that at least aportion of an optical beam transmitted through the entrance face intothe substrate is internally reflected from the reflecting face towardthe substrate upper surface. A photodetector active region is formed atthe substrate upper surface and is positioned so that at least a portionof the optical beam reflected from the reflecting face impinges on atleast a portion of the active region.

Large numbers of photodetectors thus formed may be fabricatedsimultaneously using wafer-scale spatially-selective material processingtechniques, which may be implemented by processing only a single wafersurface. Once fabricated (and separated from other photodetectors on thewafer, if wafer-scale processing is employed), a photodetector may beinverted and mounted on a planar waveguide substrate for receiving anoptical beam emerging from the end of a planar waveguide formed on thewaveguide substrate. At least a portion of the optical beam may enterthrough the entrance face, reflect from the reflecting face, and impingeon the active region. In this way a photodetector may be readilyintegrated into a composite optical device assembled on the planarwaveguide substrate. Alternatively, the photodetector substrate may beprovided with a fiber alignment groove, or may be positioned on a secondsubstrate having a fiber alignment groove, so that light emerging froman end face of an optical fiber positioned in the groove may enterthrough the entrance face, reflect form the reflecting face, and impingeon the active region.

Objects and advantages pertaining to a semiconductor photodetector withan internal reflector may become apparent upon referring to thedisclosed exemplary embodiments as illustrated in the drawings and setforth in the following written description and/or claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic diagrams of a photodetector mounted on aplanar waveguide substrate.

FIGS. 1C and 1D are schematic diagrams of a photodetector mounted on agrooved substrate with an optical fiber.

FIGS. 2A and 2B are side cross-sectional and top views, respectively, ofa photodetector with an internal reflector.

FIGS. 3A and 3B are side cross-sectional and top views, respectively, ofa photodetector with an internal reflector.

FIG. 4 is a side view of a photodetector with an internal reflector.

FIGS. 5A and 5B are side and top views, respectively, illustrating aprocess sequence for fabricating a photodetector with an internalreflector.

FIGS. 6A, 6B, 6C, and 6D illustrate a process step for fabricating aphotodetector with an internal reflector.

FIGS. 7A and 7B are side and top views, respectively, of a photodetectorwith an internal reflector.

FIG. 8 illustrates mounting of a photodetector with an internalreflector onto a planar waveguide substrate.

FIGS. 9A and 9B are side and top views, respectively, of a photodetectorwith an internal reflector.

FIG. 10 illustrates mounting of a photodetector with an internalreflector onto a planar waveguide substrate.

FIGS. 11A and 11B are side and top views, respectively, of aphotodetector with an internal reflector.

FIG. 12 illustrates mounting of a photodetector with an internalreflector onto a planar waveguide substrate.

FIG. 13 illustrates mounting of a photodetector with an internalreflector onto a planar waveguide substrate.

FIG. 14 illustrates mounting of a photodetector with an internalreflector onto a grooved substrate with an optical fiber.

FIG. 15 illustrates mounting of a photodetector with an internalreflector onto a planar waveguide substrate.

FIG. 16 illustrates mounting of a photodetector with an internalreflector onto a grooved substrate with an optical fiber.

FIG. 17 is a top view of a photodetector with an internal reflector.

FIG. 18 is a top view of a photodetector with an internal reflector.

FIG. 19 is a top view of a photodetector with an internal reflector.

It should be noted that the relative proportions of various structuresshown in the Figures may be distorted to more clearly illustrateexemplary embodiments. Relative dimensions of various optical devices,optical waveguides, optical components, alignment/support members,electrodes/contacts, and so forth may be distorted, both relative toeach other as well as in their relative transverse and/or longitudinalproportions. In many of the Figures the transverse dimension of anoptical element is enlarged relative to the longitudinal dimension forclarity, which will cause variations of transverse dimension(s) withlongitudinal position to appear exaggerated. Thicknesses of variouslayers may also be exaggerated.

The embodiments shown in the Figures are exemplary, and should not beconstrued as limiting the scope of the present disclosure and/orappended claims.

DETAILED DESCRIPTION OF EMBODIMENTS

An exemplary photodetector with an internal reflector is shown in FIGS.2A and 2B. A semiconductor substrate 302 is spatially-selectivelyprocessed to form adjacent substrate upper surface areas 301 and 303 atdiffering heights and separated by an entrance face 304. A reflectingface 306 is formed at the substrate upper surface (bordering area 303)by spatially-selective processing. An n-type semiconductor layer 310(i.e., an n-layer), an intrinsic semiconductor layer 312 (i.e., ani-layer), and another n-layer 314 are formed at the substrate uppersurface area 303. Exemplary materials for producing a III-Vsemiconductor photodetector are semi-insulating InP or n-type InP forthe substrate 302, n-type InP for the n-layers 310 and 314, and InGaAsfor i-layer 312. The upper n-layer may be spatially-selectively doped toproduce a p-type area 316 (i.e., a p-layer). Alternatively, a p-layerlayer may be initially present (instead of n-layer 314) andspatially-selectively etched to form p-layer area 316 (as shown in FIGS.3A and 3B). Layers 310, 312, and 316 from a p-i-n junction thatfunctions as the photodetector active region. Metal contact layers maybe applied, for example contact 310 a formed on an exposed portion ofn-layer 310, and contact 316 a formed on p-layer 316, thereby providingco-sided contacts to the p-i-n photodetector. Alternatively, an n-typesubstrate may be employed, and contact 310 applied to the opposite sideof the substrate 302. Incident optical power may enter the substratethrough entrance face 304 and propagate through a portion of substrate302, and at least a portion of the incident optical power may beinternally reflected from reflecting face 306 to impinge on at least aportion of the photodetector active region, generating an electronicsignal.

Any suitable type of photodetector active area may be formed whileremaining within the scope of the present disclosure and/or appendedclaims. Examples may include but are not limited to p-i-n photodiodes(photoconductive or photovoltaic), avalanche photodiodes, Schottkydiodes, phototransistors, metal-semiconductor-metal (MSM)photodetectors, combinations thereof, and/or functional equivalentsthereof. Any suitable semiconductor material or material combinationsmay be employed while remaining within the scope of the presentdisclosure and/or appended claims. Examples may include, but are notlimited to: silicon and/or silicon-based semiconductors; germaniumand/or germanium-based semiconductors; III-V semiconductors and/oralloys thereof; n-doped and/or p-doped variants thereof; combinationsthereof; and/or functional equivalents thereof.

The entrance face 304 may form an angle varying over a wide rangedepending on the desired optical configuration for the photodetector.Many useful optical configurations may be implemented with an angle αbetween about 60° and about 120°, and typical configurations may employan angle α between about 85° and about 110°. Angles between about 85°and about 110° between the entrance face 304 and the substrate uppersurface areas 301 and 303 yield incidence angles from about 5°(reflecting face surface normal below horizontal) through 0° (normalincidence) to about 20° (reflecting face surface normal abovehorizontal), for light propagating substantially parallel to theadjacent substrate surface areas 301 and 303. Other incident propagationdirections may be accommodated while remaining within the scope of thepresent disclosure and/or appended claims. The entrance face may beformed by a dry etch process (such as reactive ion etching) that enablescontrol of the vertical angle and horizontal orientation of theresulting etched face. Other etch processes allowing the angle to bechosen may be employed, or it may be possible for a given substratematerial and crystal orientation to make use of etching along crystalplanes in the material to achieve the desired entrance face angle. A sawcut or other mechanical material processing techniques could instead beemployed for forming entrance face 304. Regardless of the processingemployed for forming entrance face 304, in some cases subsequentprocessing may result in absence of substrate upper surface area 301from the finished photodetector, while in other cases the finishedphotodetector may include at least a portion of substrate upper surfacearea 301. An entrance face 304 oriented along a crystal plane mayalternatively be formed by cleaving the substrate, which would alsoeliminate substrate surface area 301 from the finished photodetector.

For an InP substrate (n≈3.2) in air and an incident beam propagatingsubstantially parallel to substrate surface areas 301 and 303, thestated range of entrance face orientations leads to a range of refractedangles from about 2° above horizontal through 0° to about 14° belowhorizontal. For refraction below horizontal, the refracted beam directeddeeper into the substrate, away from the adjacent substrate uppersurface area 303 and away from the photodetector active region (activeregion labeled 318 in FIG. 4). For an InP substrate embedded in atypical transparent “potting” or encapsulating medium (n≈1.4–1.5, forexample), the refracted optical beam forms an angle ranging betweenabout 2.3° above horizontal and about 11° below horizontal. The entranceface 304 may be antireflection coated to decrease reflective lossesand/or reduce optical feedback to upstream optical devices or components(about 27% reflection at an uncoated InP/air interface; about 14% at anuncoated InP/encapsulant interface). Non-normal incidence at entranceface 304 may also serve to reduce optical feedback to upstream opticaldevices or components arising from reflection from the entrance face. Ifthe photodetector is to be used in a multi-wavelength optical system orassembly, a wavelength-selective filter coating of any suitable type maybe formed on the entrance face, such as a long-pass, short-pass,bandpass, or notch filter.

Reflecting face 306 may be formed by a spatially-selective etch process.A wet etch process may be employed for forming reflecting face 306,which forms along a crystal plane of the substrate material. For InP(crystallographic 100 surface substantially parallel to the substratesurface), the reflecting face 306 forms at an angle between about 51°and about 60° (usually about 55°) with the substrate surface (angle β ofFIG. 4), so that an optical beam refracted at entrance face 304 (asdescribed above) and propagating within the substrate may be internallyreflected upward toward the photodetector active region 318 at thesubstrate surface. For other crystal orientations and/or other substratematerials, reflecting face 306 may form along a crystal plane at anotherangle. Use of a crystal plane for defining reflecting face 306 resultsin a reproducible face orientation and a reflecting surface of highoptical quality. Alternatively, reflecting face 306 may be formed by anyother suitable spatially-selective etch process(es), including dry etchprocesses, that form the reflecting face at an angle determined by thecrystallographic structure of the substrate, or that may form thereflecting face at any desired angle. A saw cut or other mechanicalmaterial processing techniques could instead be employed for formingreflecting face 306. The angle between reflecting face 306 and thesubstrate upper surface area 303 may typically range between about 40°and about 70°, may more typically range between about 45° and about 65°,and may range between about 51° and about 60° for many commonphotodetector implementations.

Dimensions for a photodiode with an internal reflector may be subject toa variety of practical constraints. The following are exemplarydimensions that may be used for implementing an InP-based photodetectorwith an internal reflector, but should not be interpreted as limitingthe scope of inventive concepts disclosed and/or claimed herein. Aprimary constraint is the minimum distance between an edge of thephotodetector active region 318 and the edge of the etched reflectingface 306 (dimension A in FIG. 4). Performance of the photodetector maydegrade if the photodetector active region is too close to the etchededge (less than about 5–7 μm away for a p-i-n photodetector on InP; thismay depend on material quality and/or processing quality control), withsuch detectors potentially exhibiting poor reliability and/or high darkcurrents. For a substantial portion of the optical beam to reach thephotodetector active region at this position (i.e., at least 5–7 μm awayfrom the reflector edge), an upper portion of the optical beam shouldreflect from reflecting face 306 at a depth greater than a minimum depthwithin photodiode substrate 302. This minimum depth also depends on theangle of the reflecting face. For InP with a reflecting face betweenabout 51° and about 60°, the depth of an upper portion of the reflectedoptical beam may typically be greater than about 5–7 μm. To accommodatethis distance and typical beam sizes/divergences encountered (seebelow), the overall etch depth for reflecting face 306 (dimension B inFIG. 4) should in most cases be greater than about 10 μm, and istypically between about 30 μm and about 50 μm. The size and/or positionof the incident optical beam may force this minimum etch depth to bemade larger. There may also be a practical upper limit for this etchdepth, however. The InP photodetector substrate 302 may typically bethin (perhaps as thin as about 150 μm or less). The depth of thereflecting face etch should not be too large a fraction of this overallthickness, so as too avoid excessive weakening of the substrate, andpotential device failure. Larger etch depths may also require largerareas of the substrate to be masked, decreasing the density of devicesthat may be fabricated on a single substrate wafer. The etch depth of awet etch may typically be controlled by etchant concentration and etchtime, although other techniques may be employed (see below) forcontrolling the depth of wet etch, dry etch, or other processes used forproviding reflecting face 306.

On the input side, the entrance face 304 should be etched at leastdeeply enough below the level of photodetector active area 318(dimension C in FIG. 4) to accommodate (both in terms of size andposition) an optical mode transmitted through the entrance face 304. Anoptical mode supported by a planar waveguide with a relatively smalland/or transversely asymmetric core may be only a few μm across uponexiting the waveguide and exhibit correspondingly large beam divergence.The fraction of such a divergent optical beam entering entrance face 304may therefore be limited unless the entrance face is sufficiently closeto the waveguide end face or sufficiently large to accommodate thedivergent optical beam farther from the waveguide end face.Alternatively, a larger and correspondingly less divergent optical modemay emerge from the end of a planar waveguide or an optical fiber; suchlarger modes do not typically exceed about 10 μm in transverse extent.The entrance face in this instance should be sufficiently large toaccommodate the optical mode, which may not vary much over the distancebetween the waveguide end face and entrance face 304. The depth of theentrance face 304 may be limited by the etching processes employedand/or by geometric constraints of the planar waveguide substrate (forexample if the entrance face must be positioned facing the end of theplanar waveguide while contacts on the photodetector make contact withthe waveguide substrate). A minimum etch depth for forming the entranceface 304 may be about 5 μm (suitable for a small optical mode emergingfrom a waveguide close to the entrance face, for example), while moretypical photodetectors may be fabricated with the entrance face etchedto a depth between about 30 μm and about 50 μm. An optical beamtransmitted through the entrance face 304 (once the photodetector hasbeen mounted on a second substrate with a planar waveguide or fiber) istypically centered on the entrance face between about 2.5 μm and about50 μm below the level of active area 318, often between about 10 μm andabout 20 μm below the level of the active area. Other etch depths forthe entrance face and other positions for the transmitted optical beamon the entrance face may be employed while remaining within the scope ofthe present disclosure and/or appended claims. It should be noted thatthe upper edge of the entrance face may or may not coincide with thelevel of the active area in the finished photodetector, depending on theparticular spatially selective material processing employed to form theentrance face, reflecting face, and active area.

The angle of the entrance face 304 (angles α in FIG. 4), the distancealong the substrate upper surface between the upper edges of entranceface 304 and reflecting face 306 (dimension D in FIG. 4), and the angleof the reflecting face (angle β in FIG. 4) together determine theposition on reflecting face 306 from which the optical beam isreflected. The angle α of entrance face 304 for an exemplaryphotodetector may range between about 95° and about 99°, resulting in anangle of incidence on the reflecting face between about 29° and about33° for a reflecting face angle β of about 55°. These incident anglesare above the critical angle for total internal reflection (about 18°for an air/lnP interface; about 27° for an encapsulant/InP interface),and result in a depth change at reflecting face 306 of about 5%–10% ofthe face-to-face distance. To achieve a minimum depth of at least 5–7 μmat the reflecting face (as discussed above), an optical beam may betransmitted through the entrance face at a depth greater than or equalto about 5–7 μm, or a smaller entrance face depth may be accommodated bya sufficiently large face-to-face distance. Larger entrance face depthand/or larger face-to-face distances result in greater reflecting facedepth. There may be an upper limit on the entrance face depth (asdescribed above), and there may be an upper limit on the face-to-facedistance (less than about 250 μm, for example) by the divergence of theoptical beam, the sensitivity/speed demands placed on the photodetector,size constraints on the photodetector, and any processing limits on theetch depth for reflecting face 306 (discussed above). Many typicalphotodetectors may have a face-to-face distance between about 50 m andabout 250 μm; distances outside this range may nevertheless fall withinthe scope of the present disclosure and/or appended claims.

Smaller face-to-face distances may be required when detection efficiencyis at a premium. Such may be the case: when the optical beam is moredivergent; when the incident optical signal power is small; when ahigh-speed [10 Gbit/sec or more], and therefore smaller area,photodetector is called for; and so forth. A smaller face-to-facedistance results in a larger fraction of the optical beam impinging onthe active region of the photodetector, improving overall detectionefficiency of the photodetector. In applications where detectionefficiency may not be so critical (less divergent beam, low-speeddetection, larger active area, large optical signal power, etc), largerface-to-face distance may be employed, potentially relaxing fabricationtolerances and/or improving device yields (for example, if the activephotodetector region 318 need not be quite so close to etched reflectingface 306).

An exemplary photodetector may comprise an InP substrate with a p-i-nactive area, with an active area about 15 μm wide and about 24 μm long,with about a 12 μm gap between the active area and the reflecting faceedge. The reflecting face angle (β) may be about 55°, and theface-to-face distance is about 100 μm. The entrance face angle (α) mayrange between about 95° and about 99°, and the entrance face depth(between the active area level and the center of an optical beamtransmitted through the entrance face) may be about 13.5 μm for annon-encapsulated photodetector, or about 15.5 μm for an encapsulatedphotodetector. The corresponding reflecting face depth (between theactive region level and the center of the internally-reflected opticalbeam) may be about 18 μm for a non-encapsulated photodetector and about20 μm for an encapsulated photodetector.

It should be noted that various processing steps or sequences may notproduce a sharp or well-defined edge or angle between the substrateupper surface and entrance face 304 or reflecting face 306. In somecases the edges may be unintentionally rounded or curved, or protrudingor overhanging material may be left at the edge, a protruding “foot” maybe left at the base of an etched face, and/or other irregularities maybe left after processing. In other cases one or both of the faces maynot meet the substrate upper surface by design. The angles betweensurfaces and faces referred to herein shall be angles between thoseportions of the surface or face where the intended geometry has beenachieved (e.g., substantially flat portions of a flat entrance face),regardless of whether the surfaces and faces actually meet. Similarly,the face-to-face distances referred to hereinabove shall be measuredbetween locations where the intended surfaces or faces would have met inthe absence of irregularities at the edges or processing that eliminatedthe edges.

It should be noted that while total internal reflection from reflectingface 306 is desirable for increasing the overall detection efficiency ofthe photodetector and reducing polarization-dependence of the facereflectivity, angles below the critical angle, and therefore resultingin only partial, polarization-dependent internal reflection, maynevertheless fall within the scope of the present disclosure and/orappended claims. In instances where absolute collection efficiency maynot be a critical issue, a photodetector with an internal reflector maybe implemented with only partial internal reflection from reflectingface 306. In addition, divergent optical beams propagating within thephotodetector and reflected from the reflecting face may undergo totalinternal reflection over only a portion of the divergent beam if therange of incident angles straddles the critical angle. Portions ofextremely divergent input optical beams may even impinge directly on thephotodetector active area, without undergoing internal reflection. Areflective coating of any suitable type may be formed on reflecting face306 to enhance internal reflection therefrom at any desired angle ofincidence, at the expense of extra processing steps for applying thecoating. Examples of such reflective coatings may include metalreflector coatings and multi-layer dielectric reflector coatings.

A photodetector with an internal reflector may be fabricated by thefollowing exemplary sequence of spatially selective process steps (sideview in FIG. 5A, top view in FIG. 5B). Substrate 302 may comprisesemi-insulating InP, layers 310 and 314 may comprise n-type InP, andlayer 312 may comprise a layer of semi-insulating or lightly dopedInGaAs. Masked diffusion of a p-type dopant may be employed to formp-type area 316, which may then be provided with a metal contact layer316 a. While protecting contact 316 a and p-type area 316, a portion ofn-type layer 314 and layer 312 may be removed and a metal contact 310 amay be deposited on an exposed portion of n-type layer 310. Electricalaccess is thus provided for both p-type and n-type layers 310 and 316,which with intervening layer 312 form a p-i-n photodetector activeregion. Metal electrical traces 310 b and 316 b may be deposited forenabling electrical access to contacts 310 a and 316 a, respectively.Masked dry etching may be employed for forming entrance face 304 at thedesired angle, and masked wet etching may be employed for providing thereflecting face 306 (each while protecting contacts and traces, ifformed before etching of entrance and/or reflecting faces). Allprocessing steps for forming the photodetector as well as the entranceand reflecting faces may be performed on a single semiconductorsubstrate surface, eliminating a need for processing both semiconductorsurfaces and thereby significantly reducing processing complexity andexpense. The exemplary processing sequence also yields a photodetectorhaving co-sided contacts, which may be advantageous in some instances.The process sequence may be implemented on a wafer-scale substrate formany photodiodes simultaneously. Once the processing steps arecompleted, the wafer may be divided into separate devices for deploymentand use. Many other material combinations, layer thicknesses, and/orprocessing sequences may be devised and employed for fabricating aphotodetector active region of any suitable type that nevertheless fallswithin the scope of inventive concepts disclosed and/or claimed herein.

In order to reproducibly achieve proper positioning of a wet-etchedreflecting face 306, care must be taken that the etching process doesnot undercut the mask used to define the edge of the reflecting face atthe surface of the substrate. Only if there is little or no undercuttingof the mask by the etch process will the reflecting face end up in theintended position with high optical quality substantially all the way upto the substrate surface. If the mask does not adhere sufficiently wellto the substrate and undercutting occurs, the reflecting face will endup too close to entrance face 304 and the photodiode active region(Dimensions C and D from FIG. 4 too small). This may spoil the geometryof the optical path within the photodetector substrate and decrease thefraction of incident light that reached the photodetector active region.Insufficient distance between the photodetector active region and theetched edge of reflecting face 306 may degrade the performance of thephotodetector. In the particular example of FIGS. 2A/2B, 3A/B, and5A/5B, the properties of the materials employed may be exploited formitigating this potential fabrication problem. The starting material forthe processing sequence may typically include an InP substrate with n-and/or p-doped InP layers 310 and 314 with an InGaAs intrinsic layer 312therebetween. These layers are typically epitaxially grown and are inintimate, atomic level contact with one another (interface typically oneor only a few monolayers thick). The InGaAs layer therefore may functionas an ideal mask material for a wet etch to provide reflecting surface306. Layers 310/314 and InGaAs layer 312 may be spatially selectivelyremoved from the substrate along a boundary corresponding to the desiredupper edge of reflecting face 306. The InGaAs layer is impervious to theetch and protects and constrains the upper edge of the reflecting faceas the InP substrate is etched along a crystallographic plane. Thespecific examples of substrate, etchant, and mask material(s) areexemplary. Any mask that suitably adheres to the substrate material, andany etchant that exhibits the desired crystal plane selectivity, may beequivalently employed.

If a spatially selective wet etch is employed that etches selectivelyalong two crystallographic planes, then the dimensions of a de-maskedarea may be used to determine the size (including the depth) of the wetetch. For example, In FIG. 6A a rectangular area 620 is de-masked. Anetchant is used that selectively etches along two differentcrystallographic planes of substrate 602 (for example, aqueousHBr/H3PO4applied to an InP (100) surface selectively etches along the(111 a) and (111 b) crystal planes; other etchant/crystal combinationsmay similarly exhibit such dual selectivity). FIGS. 6B, 6C, and 6D showthe results of the doubly selective etch process. A tetrahedral cavityis etched into the substrate 602 with surfaces 606 a and 606 b inclinedunder the substrate surface and each therefore able to serve as aninternal reflecting face of a photodetector. Surfaces 607 a and 607 bslope toward each other, and when they meet the etch process terminates(regardless of continued exposure to the etchant). The overall depth ofthe etch process and the precise position and dimensions of thereflecting faces are therefore determined only by the initial dimensionsand position of de-masked area 620, which may be determined accuratelyand precisely. The specific examples of substrate, etchant, and maskmaterial(s) are exemplary. Any mask that suitably adheres to thesubstrate material, and any etchant that exhibits the desired crystalplane selectivity, may be equivalently employed.

Entrance face 304 and/or reflecting face 306 may be suitably curved (inone or both dimensions) for reducing the divergence of an incidentoptical beam. Entrance face 304 may be readily provided with a lateralcurvature (as in FIG. 7A) to form a convex refracting surface bysuitable alteration of whatever spatially-selective etch process isemployed for its formation. For example, if formed by a masked etchingprocess, suitable modification of the mask may provide the desiredlateral curvature for entrance face 304. Providing a vertical curvaturefor entrance face 304 may pose a more challenging fabrication problem,but may nevertheless be employed for reducing the divergence of anincident optical beam in the vertical dimension (as in FIG. 7B).Techniques such as gray-scale lithography, for example, may yield adesirable vertical curvature for forming a convex refracting surface forentrance face 304. A suitably curved surface may be provided in asimilar manner for reflecting face 306, forming a concave internalreflection surface for reducing the divergence of an optical beampropagating from entrance face 304. Providing lateral curvature forreflecting face 306 may be readily achieved by suitable adaptation ofthe relevant spatially selective processing steps (altering a mask, forexample), while providing vertical curvature may be more problematic(particularly since reflecting face 306 is recessed relative to thesurface of the substrate). Use of an etching process restricted tocrystallographic surfaces would not be suitable for providing a curvedreflecting face 306. Use of laterally and/or vertically curved entranceand/or reflecting faces may reduce the divergence of an incident opticalbeam; may increase the fraction of an incident beam that impinges on thephotodetector active area; may enable use of longer face-to-facedistances; may enable use of smaller, faster, and/or less efficientphotodetectors; may loosen alignment tolerances between thephotodetector and the optical waveguide or fiber providing the incidentoptical beam.

Once fabricated and separated from other photodetectors on the wafer, aphotodiode fabricated with an internal reflector may be inverted andmounted for receiving light emitted from the end of a planar waveguideon a substrate (i.e., “flip-chip” mounted onto a PLC waveguide, as inthe example of FIG. 8). The substrate 501 may be provided, if necessary,with a pocket or depression for accommodating any portion of thephotodetector that may extend below the level of the planar waveguide520. A planar waveguide 520 on waveguide substrate 501 is adapted at theend thereof for emitting light propagating therethrough. The emergingoptical beam diverges as it propagates from the end of the waveguideaccording to the mode size supported by the waveguide. The output end ofthe waveguide may be adapted for mode expansion so as to decrease thedivergence of the output beam. The optical beam 513 may propagatesubstantially parallel to the waveguide substrate and enter thephotodetector through entrance face 504. After refraction at theentrance face 504, the beam is redirected to propagate deeper into thephotodetector substrate 502 (upward in FIG. 8, since the photodetectoris inverted). The optical beam is internally reflected from reflectingface 506 and directed toward photodetector active region 510.

Not shown in the Figures are alignment/support structures that may befabricated on waveguide substrate 501 and/or photodiode substrate 502for facilitating proper placement of the photodetector in waveguidesubstrate 501 substantially aligned with the end of waveguide 520 (sothat an optical beam emerging from the waveguide illuminates at least aportion of the photodetector active area). Such support/alignmentstructures may include grooves, flanges, posts, tabs, slots, yokes,solder/metal surface tension, and the like for guiding placement of thephotodetector on the waveguide substrate. Waveguide substrate 501 may beprovided with electrodes, contacts, and/or electrical traces forestablishing electrical connections to the photodetector (omitted fromthe Figures for clarity). Contacts may be incorporated intosupport/alignment structures, or may comprise separate structures.Solder or other material employed for forming electrical connectionsbetween contacts on the photodetector and mating contacts on thewaveguide substrate may also serve to mechanically bond thephotodetector to the substrate. Alternatively, the photodetector may bemechanically bonded to the waveguide substrate by a suitable adhesive.

A substantially transparent embedding medium or encapsulant 1500 maysubstantially fill the optical path between the end of planar waveguide520 and entrance face 504 of the photodetector (FIG. 15). Such asubstantially transparent embedding medium may serve to reduce unwantedreflection from the end face of the planar waveguide and fromphotodetector entrance face 504. The embedding medium may have anrefractive index near the refractive index of one of the photodetectorand planar waveguide, or between them. Any suitable embedding medium orencapsulant (substantially transparent over a desired operatingwavelength range) may be employed that reduces reflection at thewaveguide end face and photodetector entrance face relative to vacuum orambient air. The embedding medium 1500 may be spatially-selectivelyapplied between the waveguide end face and photodetector entrance face,or may instead serve to encapsulate the photodetector and the adjacentend portion of the planar waveguide, as in FIG. 15. Encapsulation ofinternal reflecting face 506 increases the critical angle for totalinternal reflection, which, if total internal reflection is desired forthe photodetector, may impose tighter ranges and/or tolerances forangular and linear dimensions of the photodetector, and may also imposetighter ranges and/or tolerances for size and divergence of an incidentoptical beam.

FIG. 14 shows an exemplary photodetector fabricated according to thepresent disclosure, including detector substrate 1402, entrance face1404, internal reflector face 1406, and photodetector active region1418, mounted on a grooved substrate 1401 (support/alignment structuresomitted for clarity). Groove 1452 is adapted for receiving an opticalfiber 1450. This exemplary assembly is similar to that of FIG. 8, withthe planar waveguide replaced by an optical fiber. Substrate 1401 isprovided with support/alignment structures (not shown) suitablypositioned so that a substantial portion of an optical beam 1413emerging from optical fiber 1450 (when positioned in groove 1452) entersentrance face 1404, reflects from reflecting face 1406, and impinges onactive region 1418. The substrate 1401 may include: a pocket or recessfor accommodating downward-protruding portions of the photodetector uponmounting; electrical contacts or traces; and/or support/alignmentstructures for mounting the photodetector on the substrate. The opticalpath between the end of the fiber 1450 and photodetector entrance face1404 may be filled with substantially transparent embedding medium orencapsulant 1600 (as described hereinabove), or the photodetector andadjacent end portion of the fiber may be encapsulated by encapsulant1600 (FIG. 16). In another exemplary embodiment (not shown), a groove isformed directly on the detector substrate 1402 and an optical fiber ismounted therein. Such an embodiment may function in a manner similarFIGS. 14 and 16, without the use of a second substrate for separatemounting of the photodetector and fiber.

Another exemplary embodiment of a photodetector with an internalreflector is illustrated in FIGS. 9A and 9B, which shows a photodetectoractive region 918 on a photodetector substrate 902, along with anyrequired electrical contacts and/or traces. The photodetector may be ap-i-n photodetector on an InP substrate as described above, or any othersuitable photodetector provided on a suitable substrate. Substrate 902is further provided with a silica-based, polymer, or other low-indexdielectric slab 912 with an entrance face 913 and an angle-etchedreflecting face 914. The angled face 914 may be fabricated at an anglesufficiently shallow for total internal reflection of light propagatingwith slab 912 downward toward substrate 902. Alternatively, angledreflecting face 914 may be provided with a reflective coating (metal,dielectric, or other) for reflecting light down toward the substrate.The angled face 914 is positioned so as to direct an optical beampropagating within slab 912 down onto photodetector 918. A interveningreflector layer 916 (metal, multi-layer dielectric, or other suitablereflector) may be employed between substrate 902 and slab 912 tosubstantially prevent leakage of light from layer 912 into substrate 902before reaching active region 918. An optical beam entering slab 912through entrance face 913 may propagate toward face 914 and be reflectedonto photodetector 918. FIG. 10 shows the photodetector of FIGS. 9A and9B inverted and flip-chip mounted on a planar waveguide substrate 1001and positioned for receiving light emerging from and end of planarwaveguide 1020 and directing the light onto photodetector 918(support/alignment structures omitted for clarity). Entrance face 913and/or reflecting face 914 may be substantially planar, or may besuitably curved in one or both dimensions so as to reduce the divergenceof an incident optical beam. The mounted photodetector embodiment ofFIG. 10 may include a substantially transparent embedding medium betweenthe waveguide and photodetector, or may be encapsulated in a mannersimilar to FIG. 15. The photodetector embodiment of FIGS. 9A and 9B mayalternatively by mounted on a substrate with an optical fiber in amanner similar to that shown in FIG. 14 or 16.

Another exemplary embodiment of a photodetector with an internalreflector is illustrated in FIGS. 11A and 11B. A photodetector activeregion 1118 is provided on photodetector substrate 1102, along with anynecessary electrical contacts and/or traces. The photodetector may be ap-i-n photodetector on an InP substrate as described above, or any othersuitable photodetector provided on a suitable substrate. A silica-based,polymer, or other low-index waveguide 1112 (of any suitable type,including a core/clad waveguide) may be fabricated on the substrate 1102and provided with an angled end-face 1114 positioned above thephotodetector active region 1118. The angled end face 1114 may befabricated at an angle shallow enough to result in total internalreflection of optical power propagating through waveguide 1112 ontophotodetector active region 1118. Alternatively, reflecting face 1114may be provided with a reflective coating (metal, dielectric, or other)for efficiently reflecting light down toward the substrate.

An input portion 1116 of waveguide 1112 may be adapted in a variety ofways for receiving optical power for detection by the photodetector.FIG. 12 shows a photodetector as in FIGS. 11A and 11B (includingsubstrate 1102, photodetector 1118, and waveguide 1112) inverted andflip-chip mounted onto a planar waveguide substrate 1201(support/alignment structures omitted for clarity). Waveguide 1220 andinput end 1116 of waveguide 1112 are adapted in this example forend-transfer of optical power therebetween, requiring sufficientlyprecise relative positioning and alignment for achieving anoperationally acceptable degree of optical power transfer. The exit faceof waveguide 1220, the entrance face of the waveguide 1112, and/or thereflecting face 1114 may be flat, or one or more of them may be suitablycurved in one or both dimensions for reducing the divergence of anincident optical beam. FIG. 13 shows a photodetector as in FIGS. 11A and11B (including substrate 1102, photodetector 1118, and waveguide 1112)inverted and flip-chip mounted onto a planar waveguide substrate 1301(support/alignment structures omitted for clarity). Waveguide 1320 andinput end 1116 of waveguide 1112 are adapted in this example fortransverse-transfer of optical power therebetween(mode-interference-coupled or substantially adiabatically coupled),requiring sufficiently precise relative positioning and alignment forachieving an operationally acceptable degree of optical power transfer(typically with tolerances relaxed relative to end-transfer). Reflectingface 1114 may be flat or suitably curved in one or both dimensions forreducing the divergence of an incident optical beam. The mountedphotodetector embodiment of FIG. 12 may include a substantiallytransparent embedding medium between fiber and photodetector, or may beencapsulated in a manner similar to FIG. 15. The mounted photodetectorembodiment of FIG. 13 may also be encapsulated in a manner similar toFIG. 15. The photodetector embodiment of FIGS. 11A and 11B mayalternatively by mounted on a substrate with an optical fiber in amanner similar to that shown in FIG. 14 or 16.

In the exemplary embodiments disclosed thus far, the entrance andreflecting faces of the photodetector have been shown substantiallyparallel to one another in the horizontal dimension (as in FIGS. 2B, 3B,5B, 7B, 9B, and 11B), and an optical beam enters through the entranceface near normal incidence in the horizontal dimension (as in FIG. 17).Redirection of the incident optical beam is primarily in the verticaldimension (as shown in FIGS. 8 and 14–16), and the point of transmissionthrough the entrance face, the point of reflection from the reflectingface, and the illuminated portion of the photodetector active area areall substantially lined up with one another in the horizontal dimension(as in FIG. 17, which shows optical beam 1701 transmitted throughentrance face 1704, reflected from reflecting face 1706, and impingingon photodetector active area 1710). The nominal planes of incidence withrespect to the entrance face and the reflecting face are the samesubstantially vertical plane in the arrangement of FIG. 17. In someinstances it may be desirable for the optical beam to be redirected inboth horizontal and vertical dimensions upon internal reflection (as inFIGS. 18 and 19). In these arrangements the respective planes ofincidence relative to the entrance face and reflecting face are notparallel, and the plane of incidence relative to the reflecting face isnot vertical. Such multi-dimensional beam redirection typically resultsin a larger angle of incidence as the optical beam impinges on thephotodetector active area, in turn resulting in an increased effectiveinteraction length through the thickness of the active area. Detectionefficiency may therefore be increased by increasing the interactionlength, and achieving this through a larger angle of incidence mayenable use of thinner (and therefore more readily and/or inexpensivelyfabricated) material layers to form the photodetector active area. Inaddition, beam redirection in both horizontal and vertical dimensionsmay allow positioning of the photodetector on a waveguide substrate atvarying orientations relative to waveguide(s) on the substrate (i.e.,some beam steering occurs within the photodetector substrate), enablingmore compact assembly of optical devices using less waveguide substratearea.

In the exemplary embodiment of FIG. 18, the entrance face 1804 andreflecting face 1806 are substantially parallel, with incident opticalbeam 1801 off-normal (horizontally) upon transmission through entranceface 1804. Refraction results in horizontal redirection of the opticalbeam and off-normal incidence (horizontally) on reflecting face 1806.Photodetector active area 1810 is positioned so as to receive at least aportion of the optical beam reflected from face 1806. The point oftransmission through face 1804, the point of reflection from face 1806,and the portion of active area 1810 illuminated by the reflected opticalbeam do not lie along a line when viewed from above, and the incidenceangle on the photodetector active area is larger than for horizontallyaligned embodiments. In the exemplary embodiment of FIG. 19, theentrance face 1904 and reflecting face 1906 are not parallel, and theincident optical beam 1901 is substantially normal (horizontally) upontransmission through entrance face 1904. Non-parallel arrangement of thefaces 1904 and 1906 results in off-normal incidence (horizontally) onreflecting face 1906. Photodetector active area 1910 is positioned so asto receive at least a portion of the optical beam reflected from face1906. The point of transmission through face 1904, the point ofreflection from face 1906, and the portion of active area 1910illuminated by the reflected optical beam do not lie along a common linewhen viewed from above, and the incidence angle on the photodetectoractive area is larger than for horizontally aligned embodiments.Additional embodiments may be implemented with both off-normal incidenceat the entrances face and non-parallel arrangement of the entrance andreflecting faces.

For purposes of the foregoing written description and/or the appendedclaims, the term “optical waveguide” (or equivalently, “waveguide” or“transmission optical element”) as employed herein shall denote astructure adapted for supporting one or more optical modes. Suchwaveguides shall typically provide confinement of a supported opticalmode in two transverse dimensions while allowing propagation along alongitudinal dimension. The transverse and longitudinaldimensions/directions shall be defined locally for a curved waveguide;the absolute orientations of the transverse and longitudinal dimensionsmay therefore vary along the length of a curvilinear waveguide, forexample. Examples of optical waveguides may include, without beinglimited to, various types of optical fiber and various types of planarwaveguides. The term “planar optical waveguide” (or equivalently,“planar waveguide”) as employed herein shall denote any opticalwaveguide that is formed on a substantially planar substrate. Thelongitudinal dimension (i.e., the propagation dimension) shall beconsidered substantially parallel to the substrate. A transversedimension substantially parallel to the substrate may be referred to asa lateral or horizontal dimension, while a transverse dimensionsubstantially perpendicular to the substrate may be referred to as avertical dimension. Examples of such waveguides include ridgewaveguides, buried waveguides, semiconductor waveguides, otherhigh-index waveguides (“high-index” being above about 2.5), silica-basedwaveguides, polymer waveguides, other low-index waveguides (“low-index”being below about 2.5), core/clad type waveguides, multi-layer reflector(MLR) waveguides, metal-clad waveguides, air-guided waveguides,vacuum-guided waveguides, photonic crystal-based or photonicbandgap-based waveguides, waveguides incorporating electro-optic (EO)and/or electro-absorptive (EA) materials, waveguides incorporatingnon-linear-optical (NLO) materials, and myriad other examples notexplicitly set forth herein which may nevertheless fall within the scopeof the present disclosure and/or appended claims. Many suitablesubstrate materials may be employed, including semiconductor,crystalline, silica or silica-based, other glasses, ceramic, metal, andmyriad other examples not explicitly set forth herein which maynevertheless fall within the scope of the present disclosure and/orappended claims.

One exemplary type of planar optical waveguide that may be suitable foruse with optical components disclosed herein is a so-called PLCwaveguide (Planar Lightwave Circuit). Such waveguides typically comprisesilica or silica-based waveguides (often ridge or buried waveguides;other waveguide configuration may also be employed) supported on asubstantially planar silicon substrate (often with an interposed silicaor silica-based optical buffer layer). Sets of one or more suchwaveguides may be referred to as planar waveguide circuits, opticalintegrated circuits, or opto-electronic integrated circuits. A PLCsubstrate with one or more PLC waveguides may be readily adapted formounting one or more optical sources, lasers, modulators, and/or otheroptical devices adapted for end-transfer of optical power with asuitably adapted PLC waveguide. A PLC substrate with one or more PLCwaveguides may be readily adapted (according to the teachings of U.S.Patent Application Pub. No. 2003/0081902 and/or U.S. application No.60/466,799, for example) for mounting one or more optical sources,lasers, modulators, photodetectors, and/or other optical devices adaptedfor transverse-transfer of optical power with a suitably adapted PLCwaveguide (mode-interference-coupled, or substantially adiabatic,transverse-transfer; also referred to as transverse-coupling).

For purposes of the foregoing written description and/or appendedclaims, “spatially-selective material processing techniques” shallencompass epitaxy, layer growth, lithography, photolithography,evaporative deposition, sputtering, vapor deposition, chemical vapordeposition, beam deposition, beam-assisted deposition, ion beamdeposition, ion-beam-assisted deposition, plasma-assisted deposition,wet etching, dry etching, ion etching (including reactive ion etching),ion milling, laser machining, spin deposition, spray-on deposition,electrochemical plating or deposition, electroless plating,photo-resists, UV curing and/or densification, micro-machining usingprecision saws and/or other mechanical cutting/shaping tools, selectivemetallization and/or solder deposition, chemical-mechanical polishingfor planarizing, any other suitable spatially-selective materialprocessing techniques, combinations thereof, and/or functionalequivalents thereof. In particular, it should be noted that any stepinvolving “spatially-selectively providing” a layer or structure mayinvolve either or both of: spatially-selective deposition and/or growth,or substantially uniform deposition and/or growth (over a given area)followed by spatially-selective removal. Any spatially-selectivedeposition, removal, or other process may be a so-called direct-writeprocess, or may be a masked process. It should be noted that any “layer”referred to herein may comprise a substantially homogeneous materiallayer, or may comprise an inhomogeneous set of one or more materialsub-layers. Spatially-selective material processing techniques may beimplemented on a wafer scale for simultaneous fabrication/processing ofmultiple structures on a common substrate wafer.

It should be noted that various components, elements, structures, and/orlayers described herein as “secured to”, “connected to”, “mounted on”,“deposited on”, “formed on”, “positioned on”, etc., a substrate may makedirect contact with the substrate material, or may make contact with oneor more other layer(s) and/or other intermediate structure(s) alreadypresent on the substrate, and may therefore be indirectly “secured to”,etc, the substrate. It should also be noted that words and phrases suchas “substrate upper surface”, “vertical”, “horizontal”, “height”,“level”, and the like, when used in describing the photodetectorsubstrate, are not intended to denote absolute directions or positionsin space, but are intended rather to denote directions or positionsrelative to the processed surface of a semiconductor substrate or wafer.The “substrate upper surface” refers to the processed substrate surface(or the surface where at least a majority of processing occurs, formingthe faces and active area); “horizontal” refers to directionssubstantially parallel to the processed surface; “vertical”, “height”,“level”, and so forth refer to the direction substantially perpendicularto the processed surface; and so on.

The phrase “operationally acceptable” appears herein describing levelsof various performance parameters of photodetectors, such as collectionefficiency, detector responsivity, detection bandwidth, and so forth. Anoperationally acceptable level may be determined by any relevant set orsubset of applicable constraints and/or requirements arising from theperformance, fabrication, device yield, assembly, testing, availability,cost, supply, demand, and/or other factors surrounding the manufacture,deployment, and/or use of a photodetector or optical assembly into whichit may be incorporated. Such “operationally acceptable” levels of suchparameters may therefor vary depending on such constraints and/orrequirements. For example, a lower collection efficiency may be anacceptable trade-off for achieving higher detection bandwidth in someinstances, while higher collection efficiency may be required in otherinstances in spite of decreased detection bandwidth. The “operationallyacceptable” collection efficiency and detection bandwidth therefore varybetween the instances. Many other examples of such trade-offs may beimagined. Semiconductor photodetectors, fabrication methods therefor,and incorporation thereof into optical devices and/or assemblies, asdisclosed herein and/or equivalents thereof, may therefore beimplemented within tolerances of varying precision depending on such“operationally acceptable” constraints and/or requirements. Phrases suchas “substantially transparent”, “substantially adiabatic”,“substantially spatial-mode-matched”, “substantially parallel”,“substantially normal incidence”, and so on as used herein shall beconstrued in light of this notion of “operationally acceptable”performance.

While particular examples have been disclosed herein employing specificmaterials and/or material combinations and having particular dimensionsand configurations, it should be understood that other suitablematerials and/or material combinations may be employed in a range ofdimensions and/or configurations while remaining within the scope ofinventive concepts disclosed and/or claimed herein.

For purposes of the present disclosure and appended claims, theconjunction “or” is to be construed inclusively (e.g., “a dog or a cat”would be interpreted as “a dog, or a cat, or both”; e.g., “a dog, a cat,or a mouse” would be interpreted as “a dog, or a cat, or a mouse, or anytwo, or all three”), unless: i) it is explicitly stated otherwise, e.g.,by use of “either . . . or”, “only one of . . . ”, or similar language;or ii) two or more of the listed alternatives are mutually exclusivewithin the particular context, in which case “or” would encompass onlythose combinations involving non-mutually-exclusive alternatives. It isintended that equivalents of the disclosed exemplary embodiments andmethods shall fall within the scope of the present disclosure and/orappended claims. It is intended that the disclosed exemplary embodimentsand methods, and equivalents thereof, may be modified while remainingwithin the scope of the present disclosure or appended claims.

1. A method, comprising: receiving into a semiconductor substratethrough an entrance face formed at a surface thereof an incident opticalbeam propagating substantially parallel to the substrate surface, andrefracting the optical beam away from the substrate surface, theentrance forming an angle with the substrate surface; reflectinginternally toward the substrate surface at least a portion of therefracted optical beam from a reflecting face formed on the substrate atthe substrate surface and forming an acute angle therewith; andreceiving a least a portion of the reflected optical beam at aphotodetector active region formed at the substrate surface.
 2. Themethod of claim 1, wherein the photodetector active region comprises ap-i-n photodiode.
 3. The method of claim 2, wherein the substratecomprises InP, and wherein the p-i-n photodiode comprises an InPn-layer, an InGaAs i-layer, and an InP p-layer.
 4. The method of claim1, wherein the photodetector active region comprises an avalanchephotodiode.
 5. The method of claim 1, wherein the entrance face forms anangle with the substrate surface greater than 90° and less than about105°.
 6. The method of claim 1, wherein the entrance face includes ananti-reflection coating thereon.
 7. The method of claim 1, wherein theentrance face includes a wavelength-selective filter coating thereon. 8.The method of claim 1, wherein the reflecting face forms an angle withthe substrate surface between about 40° and about 70°.
 9. The method ofclaim 8, wherein an incident optical beam propagating substantiallyparallel to the substrate surface and transmitted through the entranceface into the substrate undergoes total internal reflection from thereflection face.
 10. The method of claim 8, wherein the reflecting faceforms an angle with the substrate surface between about 51° and about60°.
 11. The method of claim 8, wherein the reflecting face includes areflective coating thereon.
 12. The method of claim 1, wherein thephotodetector active region and the reflecting face are separated bymore than about 5 μm at the substrate surface.
 13. The method of claim1, wherein the entrance face and the reflecting face are separated bymore than about 50 μm and less than about 250 μm at the substratesurface.
 14. The method of claim 1, further comprising transmitting anelectrical signal generated by the reflected beam received at thephotodetector active region via at least two electrical contacts formedat the substrate surface and connected to the photodetector activeregion.
 15. The method of claim 1, wherein the entrance face and thereflecting face are arranged so that the optical beam, if transmittedthrough the entrance face at normal incidence, defines a substantiallyvertical plane of incidence relative to the reflecting face.
 16. Themethod of claim 1, wherein the entrance face and the reflecting face arearranged so that the optical beam, if transmitted through the entranceface at normal incidence, defines a non-vertical plane of incidencerelative to the reflecting face.
 17. The method of claim 1, wherein thereflecting face is substantially parallel to a crystal plane of thesubstrate.
 18. The method of claim 1, wherein the entrance face or thereflecting face is curved.
 19. The method of claim 1, further comprisingreceiving at least a portion of an optical beam emerging from an endface of a transmission optical element as the incident optical beam, thetransmission optical element being positioned on a second substrate, thesemiconductor substrate being mounted on the second substrate with thesubstrate surface facing the second substrate.
 20. The method of claim19, wherein the transmission optical element is a planar waveguideformed on the second substrate.
 21. The method of claim 19, wherein thetransmission optical element is an optical fiber mounted in a groove onthe second substrate.
 22. The method of claim 19, wherein the incidentoptical beam is centered on the entrance face between about 2.5 μm andabout 50 μm below the level of the photodetector active region.
 23. Themethod of claim 19, wherein a substantially transparent embedding mediumsubstantially fills an optical path between the end face of thetransmission optical element and the entrance face.
 24. The method ofclaim 19, wherein the mounted semiconductor substrate and the end faceof the transmission optical element are encapsulated.
 25. The method ofclaim 1, further comprising receiving at least a portion of an opticalbeam emerging from an end face of an optical fiber as the incidentoptical beam, the optical fiber being mounted on the substrate in agroove formed thereon.
 26. An optical apparatus, comprising: asemiconductor substrate having a substrate surface; means formed at thesubstrate surface for receiving into the substrate an incident opticalbeam propagating substantially parallel to the substrate surface and forrefracting the optical beam away from the substrate surface; meansformed at the substrate surface and forming an acute angle therewith forreflecting internally toward the substrate surface at least a portion ofthe refracted optical beam; and means formed on the substrate surfacefor detecting at least a portion of the reflected beam incident on thedetecting means.
 27. The apparatus of claim 26, wherein the detectingmeans comprises a photodiode.
 28. The apparatus of claim 26, wherein thereceiving and refracting means comprises an entrance face forming anangle with the substrate surface greater than 90° and less than about105°.
 29. The apparatus of claim 26, wherein the reflecting meanscomprises a reflecting face forming an angle with the substrate surfacebetween about 40° and about 70°.
 30. The apparatus of claim 29, whereinthe refracted optical beam undergoes total internal reflection at thereflecting face.
 31. The apparatus of claim 29, wherein the reflectingface forms an angle with the substrate surface between about 51° andabout 60°.
 32. The apparatus of claim 26, further comprising meansformed at the substrate surface for transmitting an electrical signalgenerated by the reflected beam received at the detecting means.
 33. Theapparatus of claim 26, further comprising a transmission optical elementpositioned on a second substrate, wherein the semiconductor substrate ismounted on the second substrate with the substrate surface facing thesecond substrate so that at least a portion of an optical beam emergingfrom an end face of the transmission optical element is transmitted intothe substrate at the receiving and refracting means.
 34. The apparatusof claim 33, wherein the transmission optical element is a planarwaveguide formed on the second substrate.
 35. The apparatus of claim 33,wherein the transmission optical element is an optical fiber mounted ina groove on the second substrate.